1. Field of the Invention
This invention relates to ultrasonic diagnostic imaging, and more particularly, to an ultrasonic imaging system that produces a large compound image by correlating movement between consecutive image frames.
2. Description of Related Art
Ultrasonic imaging techniques are commonly used to produce two-dimensional diagnostic images of internal features of an object, such as a human anatomy. A diagnostic ultrasonic imaging system for medical use forms images of internal tissues of a human body by electrically exciting an acoustic transducer element or an array of acoustic transducer elements to generate short ultrasonic pulses that travel into the body. The ultrasonic pulses produce echoes as they reflect off of body tissues that appear as discontinuities or impedance changes to the propagating ultrasonic pulses. These echoes return to the transducer, and are converted back into electrical signals that are amplified and decoded to produce a cross-sectional image of the tissues. These ultrasonic imaging systems are of significant importance to the medical field by providing physicians with real-time, high resolution images of the internal features of a human anatomy without resort to more invasive exploratory techniques, such as surgery.
The acoustic transducer which radiates the ultrasonic pulses typically comprises a piezoelectric element or matrix of piezoelectric elements. As known in the art, a piezoelectric element deforms upon application of an electrical signal to produce the ultrasonic pulses. In a similar manner, the received echoes cause the piezoelectric element to deform and generate the corresponding electrical signal. The acoustic transducer is often packaged within a handheld device that allows the physician substantial freedom to easily manipulate the transducer over a desired area of interest. The transducer can then be electrically connected via a cable to a central control device that generates and processes the electrical signals. In turn, the control device transmits the image information to a real-time viewing device, such as a video display terminal. The image information may also be stored so that other physicians may view the diagnostic images at a later date.
The individual images produced by such ultrasonic imaging systems comprise discrete frames, with each such frame having a field of view limited by the relatively narrow region traversed by the ultrasonic pulses. As the transducer is manipulated along the body surface to obtain images of an adjacent region in the anatomy, each previous image is replaced on the viewing device by a new image defined by the limited field of view of the transducer. While a skilled physician can usually interpret the discrete frames in order to obtain a clear mental picture of the entire region traversed by the transducer, the discrete frames cannot be easily tiled together to produce a single, contiguous image. This can represent a significant drawback of conventional ultrasonic imaging systems, since it is not always possible for the physician to fully appreciate the overall condition of the body by consideration of the discrete frames alone. In some extreme cases, important information concerning the condition of the body tissues can be overlooked with serious potential consequences for the patient.
Previously, it has been demonstrated that a real-time compound ultrasonic image could be generated by use of so-called compound B-scanners. These B-scanners utilize a transducer mounted on an arm assembly that constrains the transducer to move along a single plane or axis. Either the arm assembly or the transducer element itself can be provided with sensing devices that track the precise position of the transducer. This positional information could then be utilized to register each of the discrete image frames together into a single composite image. An example of a compound B-scanner utilizing angular sensing devices on an arm assembly is disclosed in U.S. Pat. No. 4,431,007, to Amazeen et al., for REFERENCED REAL-TIME ULTRASONIC IMAGE DISPLAY. Despite this potential improvement in the art, conventional compound B-scanners are awkward and inflexible to operate due primarily to the relatively bulky mechanical arm assembly. Moreover, the sensing devices add significant complexity and cost to the ultrasonic imaging system. It should be apparent that the application of such prior art techniques to modern handheld ultrasonic transducers would be completely impractical in view of these significant drawbacks.
Thus, a critical need exists for a method to combine each of the discrete frames produced by an ultrasonic imaging system into a single image. The method should be compatible with modern handheld ultrasonic transducers without encumbering the handheld transducers with position sensing devices that increase the cost, weight and complexity of such imaging systems.